Process for purifying sucrose fatty acid esters having high HLB

ABSTRACT

A process for recovering, in a purified form, sucrose fatty acid esters having a high HLB included in a reaction mixture formed by a reaction of sucrose and a fatty acid alkyl ester in an organic solvent as reaction medium in the presence of a catalyst, which comprises adjusting the reaction mixture from which a part of the organic solvent may be previously removed and to which water is added to form an aqueous solution, to a neutral pH region, adding a neutral salt and sucrose to the solution to precipitate the sucrose fatty acid esters, separating and washing the precipitate with an acidic water, and subjecting the washing liquid to ultrafiltration. The concentrate obtained by ultrafiltration may be spray-dried to form a dry powder of the sucrose esters having high HLB, and the liquid obtained by separating the precipitate may be contacted with a reverse osmosis membrane to recover sucrose. According to the invention, purified sucrose fatty acid esters having high HLB can be obtained from the reaction mixture without using an organic solvent as purification solvent, while sucrose can be recovered in high yield.

BACKGROUND OF THE INVENTION

The present invention relates to the recovery of sucrose fatty acidesters having a high HLB (hydrophilic-lipophilic balance) from thereaction mixture containing them. More particularly, the presentinvention relates to an industrially useful process for purifyingsucrose fatty acid esters having a high HLB included in the reactionmixture formed by a reaction of sucrose and fatty acid alkyl esters inan organic solvent medium, while recoverying the unreacted sucrose inhigh yield.

Sucrose fatty acid esters (sugar esters) useful as surface active agentsare prepared industrially at present by either a solvent process whereinsucrose is reacted with a methyl ester of a higher fatty acid having 8to 22 carbon atoms in the presence of a suitable catalyst in an organicsolvent such as dimethylformamide or dimethylsulfoxide, as disclosed inJapanese Patent Publication Kokoku No. 35-13102; or an aqueous mediumprocess wherein sucrose is formed into a molten mixture with a fattyacid salt (soap) using water without using an organic solvent, and isthen reacted with a higher fatty acid methyl ester in the presence of acatalyst, as disclosed in Japanese Patent Publication Kokoku No.51-14485.

However, even according to any of these processes, the obtained reactionmixture contains impurities such as the unreacted sucrose, the unreactedfatty acid methyl ester, residual catalyst, soap, free fatty acid,volatile material (reaction solvent), etc. in addition to the desiredsucrose fatty acid ester. These impurities, at least impurities whosecontents exceed the specified amounts must be removed prior to being puton the market. Particularly, in case of sucrose fatty acid esters usedas a food additive which requires a high purity, removal of the solvent(volatile material) remaining in the product is very important in viewof strict regulation, e.g. provision by FDA, U.S.A. according to whichallowable content of remaining dimethylsulfoxide in sucrose fatty acidesters is at most 2 p.p.m. [Fed Regist., 51(214), 40160-1].

In general, the conversion of sucrose is low. For example, in case ofthe process using dimethylformamide as the reaction medium, theconversion is at most 50 %. Accordingly, recovery of the unreactedsucrose is also important.

In order to remove the remaining organic solvents and to recover theunreacted sucrose from the reaction mixture (namely crude sucrose fattyacid esters), various processes for the purification of crude producthave hitherto been proposed These purification processes usually requirea large amount of organic solvents (for example, butanol, toluene,methyl ethyl ketone as disclosed in Japanese Patent Publication TokkyoKokoku No. 42-11588 and No. 48-10448). However, in the production ofsucrose fatty acid esters on an industrial scale, the use of a largeamount of organic solvents has the following disadvantages: (1) risk ofexplosion and fire, (2) provision of explosion and fire prevention meansto electric devices, (3) application of closed system to productionequipment for explosion and fire prevention, (4) requirement offireproof construction for entire building by way of precaution againstexplosion and fire, (5) rise in fixed cost due to the items (2), (3) and(4), (6) rise in materials cost due to loss of solvent, (7)contamination of the product with remaining solvent, and (8) adverseinfluence on health of workers, and increase of cost resulting fromincrease in labor required for the prevention therefor.

In view of these circumstances, it has been desired to develop apurification technique capable of removing the unreacted sucrose andother impurities from the crude reaction mixture without using organicsolvents.

Thus, purification processes using no organic solvent have hitherto beenproposed For example, as representative methods, there have been known(1) a method wherein a sucrose fatty acid ester is precipitated byaddition of an acidic aqueous solution to the reaction mixture, asdisclosed in British Patent No. 809,815 and (2) a method wherein asucrose fatty acid ester is precipitated by addition of an aqueoussolution of a common neutral salt to the reaction mixture, as disclosedin Japanese Patent Publication Tokkyo Kokoku No. 42-8850.

However, these methods have disadvantages. When an acidic aqueoussolution, for example, hydrochloric acid, is added to the reactionmixture as in the method (1), the sucrose fatty acid ester immediatelydeposits, but the unreacted sucrose is easily decomposed and convertedinto glucose and fruit sugar. This cannot be avoided even if theaddition is conducted at a low temperature (e.g. 0° to 5° C).Accordingly, the recovery and reuse of the unreacted sucrose aredifficult.

The addition of an aqueous solution of a neutral salt such as sodiumchloride or Glauber's salt, as in the method (2), causes sucrose fattyacid esters to deposit rapidly. In this case, decomposition of unreactedsucrose does not occur, but the monoester which is an effectivecomponent in the product is dissolved in an aqueous phase. Consequently,not only the dissolution results in a large loss of the product, butalso it is a hindrance particularly to production of sucrose fatty acidesters having a high HLB which are recently in great demand. Usually,the sucrose esters have an HLB value of 1 to 20, and the larger the HLBvalue, the higher the hydrophilic property.

Also, in Japanese Patent Publication Tokkyo Kokai No. 51-29417, it isproposed to utilize a property that a mixture of water and a solventused for purification (hereinafter referred to as "purificationsolvent") separates into an upper light layer and a lower heavy layerGenerally, the lower layer contains a large amount of water and,therefore, the unreacted sucrose, which is hydrophilic, and a saltderived from a catalyst used in the synthesis of sucrose fatty acidesters are dissolved in the lower layer Since the upper layer containsthe purification solvent in a large quantity, compounds having a smallpolarity such as sucrose fatty acid esters, fatty acids and unreactedfatty acid methyl esters are dissolved in the upper layer. On the otherhand, the solvent used for the reaction such as dimethylsulfoxide isdissolved not only in the lower layer, but also inconveniently in theupper layer. Consequently, it is impossible to completely separate thereaction solvent only by this method. The product is contaminated with atrace amount of the reaction solvent, and the removal of such a traceamount of the solvent requires a further large amount of purificationsolvent.

In order to industrially realize the purification of crude sucrose fattyacid esters using water, it is very important that the reaction solventand the purification solvent are completely removed and moreover sucroseand the product are not lost.

Another important problems which must also be taken into considerationare recovery of the unreacted sucrose and drying of wet product,incident to the use of water as a purification solvent.

Since the purification of the reaction mixture with the use of water isbased on difference in water solubility between a sucrose fatty acidester and unreacted sucrose, migration of a large amount of unreactedsucrose into an aqueous phase cannot be avoided. The manufacture ofsucrose fatty acid esters cannot be industrially accepted unless such adissolved sucrose is recovered. Accordingly, it is very important toefficiently recover the sucrose which has transferred into an aqueousphase upon purification.

The water-containing sucrose fatty acid ester which has been separatedfrom the reaction mixture and to be dried, is usually in the form of anaqueous solution when the water content is over 80 % by weight, and isin the form of a slurry when the water content is less than 80 % byweight. In general, an aqueous solution of a sucrose fatty acid estershows a peculiar viscosity behavior such that the viscosity rapidlyincreases from about 40° C., reaches maximum at about 50° C. and rapidlydrops over 50° C. Some problems are encountered in removing water fromthe sucrose fatty acid ester in the form of an aqueous solution orslurry. The evaporation of water by heating under vaccum, for example,using a usual agitated vacuum dryer, is practically difficult because ofmarked foaming. Moreover, when the evaporation is conducted at a hightemperature and the contacting time with a heating source is long, notonly the sucrose fatty acid ester is decomposed, resulting in markedcoloration or caramel formation, but also the acid value is raised byfree fatty acid formed by decomposition, as disclosed in Japanese PatentPublication Tokkyo Kokoku No. 37-9966. In particular, due to theproperty of sucrose fatty acid ester that the softing point or meltingpoint is low (for example, sucrose monostearate having a melting pointof about 52° C., and sucrose distearate having a melting point of about110° C.), the sucrose fatty acid ester itself tends to be hydrated atthe final stage of evaporation of water. This makes the dehydration moredifficult. In addition, it is also a cause which make the dryingdifficult that the latent heat of evaporation of water is very high(more than 500 kcal/kg H.sub. 2O) and the evaporation temperature ishigh.

Other usual drying methods are also not suitable for preparing drysucrose fatty acid esters. For example, in case of using a flash dryerwherein a slurry is continuously heated, fed to a vacuum chamber andreleased thereto, various difficulties are encountered when a sufficientdrying is desired because of a large latent heat of water. Even if thesedifficulties are overcome, the sucrose ester dehydrated and dried undervaccum is in the molten state and, therefore, it requires apulverization step after taking out of the drier and cooling to lessthan the melting point to solidify, for instance, by blowing a cold air.In addition to many steps being required, there is a risk of dustexplosion in the final pulverization step.

Accordingly, it is also important to solve the problems encounted bydrying in realizing the purification of sucrose fatty acid esters usingwater as the purification solvent.

It is a primary object of the present invention to provide a process forrecovering a purified sucrose fatty acid ester having a high HLB fromthe reaction mixture without using organic solvents as the purificationsolvent.

A further object of the invention is to provide a process for recoveringa sucrose fatty acid ester free from organic solvents from the crudereaction mixture, with recovery of unreacted sucrose in high yield.

A still further object of the invention is to provide an industriallyuseful process for purifying a sucrose fatty acid ester using water asthe purification solvent without substantial loss of the sucrose fattyacid ester and sucrose.

Another object of the invention is to provide a process for preparing adry powder of a highly pure sucrose fatty acid ester having a high HLBwith ease and without deteriorating the quality in the drying step,while recovering the unreacted sucrose.

These and other objects of the present invention will become apparentfrom the description hereinafter.

SUMMARY OF THE INVENTION

The present inventors have made experiments about salting out in thepurification of crude product using water as the purification medium inorder to achieve the following purposes namely (1) minimizing the amountof sucrose fatty acid esters dissolved in an aqueous phase, and ifpossible, decreasing it up to zero so as to precipitate the whole ofsucrose fatty acid esters, (2) preventing decomposition of unreactedsucrose, (3) separating the reaction solvent from the sucrose fatty acidesters by dissolving the remaining reaction solvent in the aqueousphase, (4) purifying the precipitated sucrose fatty acid esters andforming a dry powder thereof, and (5) efficiently recovering theunreacted sucrose from the filtrate (or supernatant) obtained byremoving the above-mentioned precipitate.

It has been found that when sucrose and a neutral salt are dissolved inan aqueous solution of the reaction mixture, the whole of the sucrosefatty acid esters is precipitated under a proper combination of pH,temperature, concentrations of neutral salt and sucrose, and amount ofwater, and moreover, surprisingly, the reaction solvent and a saltderived from the reaction catalyst are dissolved in the aqueous phasewith the unreacted sucrose. Thus, on the basis of this dicsovery, it hasnow been found that the remaining volatile material (remaining reactionsolvent) can be transferred completely into the aqueous phase, withoutsubstantial loss of sucrose fatty acid esters, by repeating the saltingout procedure, namely by dissolving the precipitated sucrose fatty acidesters again in water and repeating the precipitation procedure by theaddition of an aqueous solution of the neutral salt and sucrose, andthat the unreacted sucrose can be efficiently recovered from theresidual liquid after removal of the above precipitate by contacting itwith an adequate reverse osmosis membrane.

It has been further found that sucrose fatty acid esters having a highHLB included in the precipitate is transferred into an aqueous phasewith soluble impurities by washing the precipitate with an acidic waterhaving an appropriate pH, while leaving sucrose fatty acid esters havinga low HLB as the solid, and that the recovery of sucrose fatty acidesters having a high HLB transferred into the aqueous phase, which hasnot been achieved by conventional processes, can be made on anindustrial scale by means of ultrafiltration to give an aqueous solutionof purified sucrose esters and from which a powder can be obtainedwithout deterioration of the quality by spray drying.

In one of the aspects of the present invention, there is provided aprocess for purifying sucrose fatty acid esters having a high HLBincluded in a reaction mixture formed by a reaction of sucrose and afatty acid alkyl ester in an organic solvent, said reaction mixturecontaining sucrose fatty acid esters, unreacted sucrose, unreacted fattyacid alkyl ester, a reaction catalyst, a soap, a fatty acid and theorganic solvent, which comprises adjusting the reaction mixture to aneutral pH region, adding water, a neutral salt and sucrose to thereaction mixture to precipitate the sucrose fatty acid esters,separating the resulting precipitate, washing the precipitate with anacidic water, neutralizing the washing liquid and subjecting it toultrafiltration.

In another aspect of the present invention, there is provided a processfor preparing a powder of sucrose fatty acid esters having a high HLBfrom a reaction mixture formed by a reaction of sucrose and a fatty acidalkyl ester in an organic solvent, said reaction mixture containingsucrose fatty acid esters, unreacted sucrose, unreacted fatty acid alkylester, a reaction catalyst, a soap, a fatty acid and the organicsolvent, which comprises adjusting the reaction mixture to a neutral pHregion, adding water, a neutral salt and sucrose to the reaction mixtureto precipitate the sucrose fatty acid esters, separating the resultingprecipitate from the aqueous phase, washing the precipitate with anacidic water, neutralizing the washing liquid and subjecting it toultrafiltration, and spray-drying the resulting concentrate.

In still another aspect, from the aqueous phase obtained by separationof the precipitate after salting out, sucrose is recovered by subjectingthe aqueous phase to reverse osmosis.

Thus, according to the present invention, from the reaction mixture, itis now possible (1) to remove the impurities, (2) to recover unreactedsucrose, (3) to obtain a powder of purified sucrose fatty acid estershaving a high HLB and (4) to separate SE product into sucrose fatty acidesters having a high HLB and those having a low HLB, without usingorganic solvents on an industrial scale.

BRIEF DESCRIPTION OF THIS DRAWINGS

FIG. 1 is a triangular graph showing the relationship between the amountof each of water, total sucrose and total salt, and the amount ofsucrose fatty acid ester dissolved in aqueous phase; and

FIG. 2 is a triangular graph showing the relationship between thesubstitution degree of sucrose fatty acid esters and the cake-liquidequilibrium in acidic water.

DETAILED DESCRIPTION

The process of the present invention is applicable to reaction mixturesobtained in the synthesis of sucrose fatty acid esters (hereinafterreferred to as "SE") by a known reaction using an organic solvent asreaction solvent.

In the synthesis of SE using an organic solvent as reaction medium,generally the solvent, for example, dimethylsulfoxide is added to amixture of sucrose and a fatty acid methyl ester in an amount of severaltimes the total amount of sucrose and the fatty acid methyl ester todissolve them. They are reacted in the presence of an alkaline catalystsuch as potassium carbonate (K.sub. 2CO.sub. 3) under reduced pressureof 20 to 30 Torrs at a temperature of 80° to 90° C. for several hours,whereby the SE is easily produced in a conversion of at least 90 %(based on the fatty acid methyl ester).

In order to deactivate the alkaline catalyst, e.g. K.sub. 2CO.sub. 3,included in the resulting reaction mixture, an equivalent amount of anorganic acid such as lactic acid or acetic acid, or a mineral acid suchas hydrochloric acid or sulfuric acid is added to the reaction mixture.By this neutralization, the catalyst changes to a corresponding salt,e.g. a potassium salt such as potassium lactate, according to the kindof the acid used in the neutralization.

The reaction solvent (e.g dimethylsulfoxide) is then distilled awayunder vacuum. The thus obtained residue (reaction mixture aftersubjected to the neutralization and vaporization of solvent) hasapproximately the following composition.

    ______________________________________                                        Ingredients         % by weight                                               ______________________________________                                        SE                  15.0 to 74                                                Unreacted sucrose   1.0 to 80                                                 Unreacted fatty acid methyl ester                                                                 0.5 to 10                                                 Salt derived from K.sub.2 CO.sub.3                                                                0.05 to 7                                                 Soap                1.0 to 10                                                 Fatty acid          0.5 to 10                                                 Volatile material (remaining                                                                      3.0 to 30                                                 reaction solvent)                                                             ______________________________________                                    

In that case, the proportion of the monoester in the SE is from 10 to 75% by weight and the proportion of di- and higher esters is from 90 to 25% by weight

Also, the acid radical mainly included in each of the fatty acid methylester, soap and fatty acid is usually a saturated acid radical having 16to 22 carbon atoms common to them.

To the reaction mixture from which the solvent is partly distilled awayand which has the above-mentioned composition, water is added in awater/reaction mixture ratio of 5 : 1 to 40 : 1 by weight, preferably 20: 1 by weight, while the pH is adjusted to 6.2 to 8.2, preferably 7.5.

When the ratio of water to the reaction mixture is less than 5, theviscosity of the obtained aqueous solution is high and the followingprocedures become difficult. Also, when excess water is added to thereaction mixture such that the weight ratio of water to the reactionmixture exceeds 40, the viscosity of the obtained aqueous solution islow and accordingly the following procedures become easy and also thedesired removal of reaction solvent can be made well, but a large energycost is required in removing water upon recovery of unreacted sucrose,etc., thus the economy is impaired.

In order to prevent decomposition of the desired SE, it is preferable toadjust the aqueous solution of the reaction mixture to pH of 6.2 to 8.2.When the pH is more than 8.2, there is a possibility that SE isquantitatively hydrolyzed by an alkali. On the other hand, even in aweak acidic region of less than pH 6.2, there is a fear of acidhydrolysis of SE, for example, when it is exposed to a high temperatureover 90° C.

To the thus pH-adjusted aqueous solution of the reaction mixture areadded a neutral salt and sucrose, preferably with keeping at atemperature of 50° to 80° C. in order to salt out SE rapidly. In thatcase, it is preferable that the neutral salt to be added satisfies thefollowing equation (1): ##EQU1## wherein the total amount of salts meansthe sum of the neutral salt to be added and the salt formed byneutralization of the catalyst used in the production of SE, and thetotal amount of sucrose means the sum of the sucrose to be added and theunreacted sucrose included originally.

Also, it is preferable to add sucrose in an amount satisfying thefollowing equation (2): ##EQU2##

In addition to the above equations (1) and (2), it is also preferablethat the total amount of salts and the total amount of sucrose satisfythe following equation (3): ##EQU3##

Any of neutral salts can be used so long as they are soluble in waterand nontoxic. Representative examples of the neutral salt are, forinstance, sodium chloride, Glauber's salt (Na.sub. 2SO.sub. 4·10H.sub.2O), potassium lactate and potassium acetate.

It has been found that when the aqueous solution containing SEprecipitate obtained by adding neutral salt and sucrose so as to satisfythe equations (1), (2) and (3) is heated to a temperature of 50° to 80°C., approximately the whole amount of SE is precipitated even if thecontent of the volatile material (remaining reaction solvent) in thereaction mixture widely ranges from 3.0 to 30.0 % by weight, and 1regardless of the kinds of the above-mentioned neutral salts to beadded. This is a peculiar phenomenon and is of important value inconnection with the objects of the present invention. By utilizing thisphenomenon, SE can be separated in the form of a slurry or cake from thewhole sucrose (unreacted sucrose and the sucrose added for theprecipitation), the volatile material, the salt derived from thecatalyst and the neutral salt which have transferred to the aqueousphase. Since the aqueous phase is not acidic, sucrose is not decomposedand, therefore, it can be recovered and reused, as occasion demands.

FIG. 1 is a triangular graph showing the above-mentioned phenomenon inmore detail When Y (g) is the weight of SE dissolved in an aqueous phaseand X (g) is the weight of SE precipitated, the weight percentage (φ%)of SE dissolved in the aqueous phase based on the total SE (X+Y) isshown by the following equation: ##EQU4## The change in the value φunderthe following conditions is shown in the accompanying drawing.

    ______________________________________                                        Conditions                                                                    ______________________________________                                        Temperature = 80° C.                                                   pH = 7.5                                                                      Water/reaction mixture = 7.4/l by weight                                      Fatty acid radical: stearic acid                                              ______________________________________                                        Composition of reaction mixture                                                                     (% by weight)                                           ______________________________________                                        SE                    29%                                                     Unreacted sucrose     35%                                                     Unreacted fatty acid methyl ester                                                                   2%                                                      Salt derived from catalyst                                                                          1%                                                      Soap                  3%                                                      Fatty acid            1%                                                      Volatile material (remaining reaction                                                               29%                                                     solvent)                                                                      ______________________________________                                        Composition of SE     (% by weight)                                           ______________________________________                                        Monoester             73%                                                     Di- and higher esters 27%                                                     ______________________________________                                    

In the drawing, the total salt and the total sucrose are as definedabove, and the sum of water, total salt and total sucrose is 100 % byweight.

The shaded part in FIG. 1 shows the region simultaneously satisfying theequations (1), (2) and (3) which has been discovered by the presentinventors.

By adding the determined amounts of sucrose and neutral salt to bedissolved in the aqueous solution of the reaction mixture so as to fallwithin the shaded region, approximately the whole amount of SE can beprecipitated, and by separating the precipitate from the aqueous phase,for example, by means of filtration or centrifugation, the volatilematerial (remaining reaction solvent), salt derived from catalyst,neutral salt added and the whole sucrose dissolved in the aqueous phasecan be completely removed.

It is also important to selectively separate and recover only sucrose(unreacted sucrose and the sucrose added for salting cut) from the thustreated aqueous solution from which the precipitated SE has beenremoved, thus which contains sucrose separated with water, the saltderived from catalyst (K.sub. 2CO.sub. 3), the neutral salt added forsalting out and the volatile material. The present inventors have foundthat utilization of a reverse osmosis membrane is particularly effectivefor this purpose. After separating the precipitated SE in a usualmanner, for example, by filtration, the filtrate is subjected to reverseosmosis.

It is expected that if a fractionation molecular weight ranging from 130to 200 is selected as that of the reverse osmosis membrane, theunreacted sucrose (molecular weight: 342) and the SE (molecular weight:more than 600) which has incidentally leaked into the aqueous phase inthe prior steps such as the salting out step, would be filtered offwithout any problem by the reverse osmosis treatment. On the other hand,substances having a molecular weight less than the fractionationmolecular weight of 130 to 200, namely the salt derived from catalystsuch as potassium lactate (molecular weight: 128), the neutral saltadded, and the volatile component such as dimethylsulfoxide (molecularweight: 78) would pass through fine pores of the reverse osmosismembrane without any problem.

As a result of conducting a large number of experiments on the basis ofthe above presumption, it has been found that when an aqueous solutioncontaining sucrose, the salt derived from catalyst, the neutral saltadded in the salting out step and the volatile material, and sometimesfurther a slight or trace amount of SE, is brought into contact with areverse osmosis membrane having a fractionation molecular weight ofabout 150 to about 200 at a temperature of 40° to 60° C. under apressure, the salt derived from catalyst, the neutral salt and thevolatile material easily pass with water through fine pores of themembrane. By this reverse osmosis procedure, low molecular weightsubstances such as the salts, volatile material and water are separatedfrom the impure aqueous sucrose solution (which may contain a slightamount of SE) to thereby form the concentrated aqueous solution of crudesucrose. An aqueous sucrose solution having a higher purity can beobtained by dissolving the concentrate in fresh water again andsubjecting the solution to the reverse osmosis treatment in the samemanner, and if necessary, further repeating these procedures.

The temperature of the aqueous solution to be fed to the reverse osmosisis important for obtaining a good result. If the temperature is lowerthan 40° C., the treating ability is remarkably lowered. Accordingly, itis desirable to select a temperature over 40° C. from a practical pointof view. On the other hand, it is advisable to conduct the treatment ata temperature below 60° C., since there is a possibility that the heatresistance of the reverse osmosis is changed at a temperature over 60°C. The pH of the aqueous solution to be treated is also important, andthe pH ranging from 6.2 to 8.2 is preferred because a fear of influenceon the quality of sucrose is minimized.

Recently, various reverse osmosis membranes have been put on the marketfrom various companies. Among them, for instance, reverse osmosismembranes of polyamide, crosslinked polyamide or polyether haveexcellent properties such as durability, heat resistance, acidresistance and alkali resistance, fungus resistance and pressureresistance. Such membranes are commercially available, for example,under a trade mark "SU-200" from Toray Engineering Kabushiki Kaisha,which has a fractionation molecular weight of about 200 and is suitableto attain the objects of the invention.

In the case of using the reverse osmosis membrane with the fractionationmolecular weight of about 200, the treatment of the aqueous solution canbe achieved with an industrially acceptable capacity by adjusting theupper limit of the concentration of the solute in the aqueous solutionto be supplied to the membrane to about 10-20 % by weight, preferablyabout 15 % by weight.

When the solute concentration is more than 20 % by weight, it isdifficult to pass water, the salt derived from the catalyst and thevolatile material through fine pores of the membrane, and accordingly itis obliged to increase a pressure to be applied as the actuation forcefor reverse osmosis, thus resulting in increase of the area of thereverse osmosis membrane. This is also very uneconomical because ofnecessity of great electric power. On the other hand, when the aqueoussolution contains the solute in a concentration of not more than about10-15 % by weight, it is sufficiently possible to industrially isolatesucrose.

For example, when passing an aqueous solution having the compositionshown in Table 1 through the reverse osmosis membrane "SU-200" with aneffective area of 8 m.sub. 2 per unit at 50° C. and pH 7.5 and under apressure applied as the actuation force for reverse osmosis of 56.0kg/cm.sup. 2G, the sucrose isolation velocity of 7.3 kg/hour is achievedOther reverse osmosis membranes similar to "SU-200", commerciallyavailable from companies other than Toray Engineering Kabushiki Kaisha,also give similar results.

                  TABLE 1                                                         ______________________________________                                        Components           Weight (kg)                                              ______________________________________                                        Sucrose fatty acid ester (stearate)                                                                0.4                                                      Sucrose              39.0                                                     Potassium lactate    9.0                                                      Volatile material (reaction solvent)                                                               5.0                                                      Soap and fatty acid  0.1                                                      Subtotal             53.5                                                     Water                481.0                                                    Total                534.5                                                    ______________________________________                                    

Like this, by repeating the reverse osmosis membrane treatment, the saltderived from the catalyst, the added neutral salt and the volatilematerial are sufficiently removed from the aqueous solution. The thusobtained aqueous solution containing sucrose can keep a sucroseconcentration of about 15 to 20 % by weight. It is economicallydisadvantageous as well as technical difficulty to obtain the aqueoussolution of sucrose with a concentration of more than 20 % by weight bythe reverse osmosis means. Accordingly, when it is desired to obtain theaqueous solution of sucrose having a sucrose concentration of more than20 % by weight, the solution is concentrated by using a usualconcentration apparatus such as a multiple effect evaporator to thedesirable concentration such as not less than 50 % by weight. Thusrecovered sucrose can be reused to the preparation of SE as a rawmaterial or used for other purposes.

The SE precipitated and separated in the salting out step is in the formof a slurry. It still contains a slight amount of impurities such as thevolatile material, salts and sucrose. These impurities can be easilyremoved by treating the slurry with an acidic water.

An aqueous slurry or cake of crude SE obtained in the salting out stepis washed with an aqueous solution of an acid having a pH of 3.0 to 5.5.The acid is not particularly limited. Preferable examples of the acidare, for instance, a mineral acid such as hydrochloric acid or sulfuricacid, and an organic acid such as acetic acid or lactic acid.Preferably, the acid solution is kept at a temperature of 10° to 40° C.By this treatment, the impurities such as volatile material, sucrose,neutral salt and catalyst-derived salt can be transferred into theacidic water. When the temperature of the acidic water is higher than40° C., the viscosity rises to hinder the operation in addition to afear of acid decomposition of SE if the operation for a long period oftime, for example, over several months, is required. On the other hand,it is uneconomical to keep the acidic water at a low temperature lowerthan 10° C., because a cooling device is required therefor. Accordingly,the acid solution treatment is effected usually at a temperature of 10°to 40° C., preferably at ordinary temperature.

Only the above-mentioned four components included in the cake or slurry,namely volatile material (reaction solvent), sucrose, neutral salt andsalt formed by neutralization of the catalyst, should be removed fromthe cake or slurry by the acid treatment. Accordingly, it is desirablethat the SE cake or slurry is in the form of particles as small aspossible so that the impurities are easily released or eluted into waterupon washing with acidic water. This can be efficiently attained byconducting the washing in a device having an ability to break into smallparticles, for example, a mixer (such as "homomixer" made by TokushuKiki Kogyo Kabushiki Kisha), a homogenizer, or a colloid mill, wherebysubstantially the whole amounts of the above-mentioned impuritiesincluded in the SE cake or slurry can be transferred into the acidicwater.

In the washing of the precipitate with an acidic water, there isobserved a noticeable phenomenon that SE having a high HLB (hereinafterreferred to as "high HLB-SE") included in the precipitate begins todissolve into the acidic water. The solubility of high HLB-SE variesdepending on the temperature, pH, etc. of the system. For example, whenthe temperature and pH of the system are ordinary temperature and 3.5,respectively, the solubility is as shown in FIG. 2.

The high HLB-SE has a high solubility in water, and here it is referredto as "water-soluble SE" and assigned with mark "Y". Since Y has a highHLB and accordingly a high water solubility, it does not precipitateeven in the acidic aqueous solution and is present therein in adissolved state. In contrast, SE having a low HLB (hereinafter referredto as "low HLB-SE") has a low water solubility, and in general it tendsto deposit in an acidic water having a certain acidity. Here, the lowHLB-SE is referred to as "depositable SE"and assigned with mark "X".Since X has a low HLB, it is apt to deposit from an acidic aqueoussolution thereof.

FIG. 2 shows a part of a triangular graph wherein the total ofmonoester, diester and triester is 100 %. In FIG. 2, point M indicatesthe composition of an original sample SE, point X indicates thecomposition of the depositable SE which has a low HLB, and point Yindicates the composition of the water-soluble SE which has a high HLB.Also, suffixes 1, 2 and 3 attached to M, X and Y show SE having adifferent proportions of sucrose esters (different ester distribution).

For example, in FIG. 2, when an aqueous acid solution of pH 3.5 is addedto a SE sample M.sub. 2 cosisting of 73 % by weight of monoester, 22 %by weight of diester and 5 % by weight of triester, the SE is dividedinto a dipositable SE (X.sub. 2) consisting of 68 % by weight of themonoester, 25 % by weight of the diester and 7 % by weight of thetriester, and the water-soluble SE (Y.sub. 2) consisting of 84 % byweight of the monoester, 13 % by weight of the diester and 3 % by weightof the triester.

The weights WX.sub. 2 of X.sub. 2 and WY.sub. 2 of Y.sub. 2 divided fromM.sub. 2 are obtained by solving the following equations (a) and (b):##EQU5## wherein Y.sub. 2M.sub. 2 is the distance between point M.sub. 2and point Y.sub. 2 , X.sub. 2M.sub. 2 is the distance between pointX.sub. 2 and point M.sub. 2, WM.sub. 2 is the weight of M.sub. 2WX.sub.2 is the weight of X.sub. 2, and WY.sub. 2 is the weight of Y.sub. 2,provided that the weights are those of the dried matters.

Like this, SE having a relatively high monoester content (namely SEhaving a high HLB) is easy to dissolve into the acidic water, whereas SEhaving a relatively low monoester content (namely SE having a low HLB)is easy to present on the precipitate side. By utilizing this property,the SE included in the reaction mixture can be quantitatively dividedinto a high HLB-SE and a low HLB-SE. There has also been found atendency that in general, the higher the monoester content in SE, themore increased the amount of SE (Y) dissolved in water be, and in thereverse case, the amount of SE (Y) dissolved in water is decreased. Withrespect to how much SE having a certain composition is dissolved in anacidic water, it can be quantitatively determined by substituting thedata φ shown in FIG. 1 in the equations (a) and (b) to give the valuesWX and WY.

Since the aqueous acid solution obtained in the washing step contains arelatively large amount of high HLB-SE, it is separated from theprecipitated SE composed mainly of low HLB-SE in a usual manner such asfiltration or centrifugation. The obtained filtrate or supernatantcontains, in addition to the high HLB-SE, small amounts of the volatilematerial (such as dimethylsulfoxide), salts and sucrose and, therefore,it is necessary to further purify the SE.

By many experiments we have found that ultrafiltration is suitable forremoving these impurities from high HLB-SE in the filtrate orsupernatant

It is known that sucrose fatty acid ester molecules aggregate with eachother to form high molecular weight micelles under certain conditions inthe aqueous solution.

Sucrose monoester, diester and triester are compounds wherein 1, 2 or 3fatty acid residues are attached to any of oxygen atoms of the 3 primaryhydroxyl groups of sucrose molecule, respectively. As well known, sincethe monoester is low in ability to form micelles in water while having alarger hydrophilic property than diester and triester, it forms arelatively low weight micelle (in other words, a micelle having a smalldiameter). In contrast, the diester and triester have a very largemicelle forming ability while being relatively low in hydrophilicproperty and, therefore, they form micelles of very large weight (namelylarge micellar diameter). It is seldom that the sucrose fatty acid esteris produced and put on the market in the form of monoester alone.Commerially available sucrose fatty acid esters are usually thoseproduced in the form of a mixture having a monoester content of 70 %, 50%, 30 % or the like.

According to the investigation of the present inventors, since highHLB-SE having a high monoester content, for instance, as high as 70 %,forms micelles of a lower weight in comparison to low HLB-SE having amonoester content as low as 50 %, the microscopic diameter of micelle issmall as much and it has a tendency to pass through a ultrafiltrationmembrane having a specified pore diameter in comparison with low HLB-SEhaving a monoester content of 50 %. Therefore, high HLB-SE having a highmonoester content has an undesirable tendency to pass through themembrane together with the unreacted sucrose, a salt formed byneutralization of a reaction catalyst with an acid, and a volatilematerial. Such a problem can be easily eliminated by selecting theultrafiltration membrane, and it is necessary for recovery of highHLB-SE to select the membrane having a low fractionation molecularweight (namely small pore diameter) when desired to remove sucrose,salts and volatile material from high HLB-SE having a high monoestercontent, and it is necessary for recovery of low HLB-SE having a lowmonoester content to select the membrane having a large fractionationmolecular weight (namely large pore diameter).

It is confirmed by the present inventors that it is practicallyimpossible to separate the unreacted fatty acid ester such as fatty acidmethyl ester, soap and fatty acid among substances included in thereaction mixture from SE by a filtration means because they are presentin the state of being included in the micelles of SE. In other words,impurities permeable together with water to a filtration membrane havingan appropriate fractionation molecular weight by a given pressure asactuating force are the sucrose (unreacted sucrose and sucrose for saltout), a salt derived from the catalyst, a neutral salt added and avolatile material (polar materials having a high affinity for sucroseand a high affinity for fatty acid ester which have been used asreaction solvents in synthesis of SE, e.g. dimethylsulfoxide anddimethylformamide), while the unreacted fatty acid ester, soap and freefatty acid are entrapped in the sucrose ester micelles and they are notimpermeable to the filtration membrane.

Thus, in this ultrafiltration step, by skillfully utilizing these factsand by selecting a filtration membrane having an appropriatefractionation molecular weight, the sucrose, salts and volatile materialare removed together with water from other components, namely SE,unreacted fatty acid ester, soap and fatty acid.

In order to select a ultrafiltration membrane having an adequatefractionating molecular weight, it is necessary to previously knowapproximate molecular weights of the subject substances. The molecularweights of typical single compounds involved in the present inventionare defined approximately as follows:

(1) Sucrose =342

(2) Unreacted fatty acid methyl ester

(main) Methyl stearate =290

(3) Salt produced by neutralization of catalyst (K.sub. 2CO.sub. 3)

In case of lactic acid: potassium lactate =128

In caser of acetic acid: potassium acetate =98

(4) Volatile material

Dimethylsulfoxide =78

Dimethylformamide =73

(5) Sucrose fatty acid ester (single compound not forming micelle)

(main) Sucrose monostearate =600

(main) Sucrose distearate =858

(main) Sucrose tristearate =1116

Other sucrose fatty acid esters such as myristate, palmitate, arachateand behenate have also similar molecualr weights to the above molecularweights.

(6) Soap

(main) Sodium stearate =298

(main) Potassium stearate =314

(7) Fatty acid

(main) Stearic acid =276

(8) Water =18

On the supposition that the apparent molecular weight of a sucrose fattyacid ester micelle may be approximately estimated as follows, if 10molecules associate per micelle, the apparent molecular weight of themicelle is:

molecular weight of monoester (600) ×10 =6,000 (regarded as 100 %monoester),

molecular weight of diester (858) ×10 =8,580 (regarded as 100 %diester), and

molecular weight of triester (1,116) ×10 =11,160 (regarded as 100 %triester).

Since the actual SE is a mixture composed mainly of mono-, di- andtriesters, the apparent molecular weight of a SE micelle is defined asthe average value thereof.

The selection of a membrane for ultrafiltration adequate for thepurposes of the present invention is conducted as follows:

In case of a ultrafiltration membrane having a fractionating melecularweight of 200, even if it is attempted to remove the unreacted sucrose,the salts such as K.sub. 2CO.sub. 3 and the volatile material (organicsolvent) while feeding the washing liquid obtained in the previous stepwith applying a pressure, the components separable by such a membraneare only water, the salts and the organic solvent which have lowermolecular weights than the fractionating molecular weight 200 of themembrane. Since sucrose which has a molecular weight of 342 larger thanthe fractionating molecular weight 200, is impermeable to the membrane,it cannot be separated and removed from SE.

In case of a ultrafiltration membrane having a fractionating molecularweight of 5,000, sucrose, salts and volatile material can easily passthrought fine pores of the membrane, since they have a molecular weightless than 5,000. SE forms micelles as mentioned above, and accordinglyit is estimated to have an apparent molecular weight of 6,000 or more onthe assumption that the number of sucrose ester molecules associated maybe 10 or more. Therefore, the micelles would not be permeable to themembrane having a fractionating molecular weight of 5,000. Since theapparent molecular weight of the micelle would be in fact more than6,000, a membrane having a fractionating molecular weight of more than5,000 can be used and this is experimentally confirmed by the presentinventors.

Investigation has been made also with respect to a membrane having afractionating molecular weight of 1,000. The results are as expected,and such a membrane can be used in the present invention.

The ultrafiltration membrane is selected from those having afractionation molecular weight of 1,000 or more in view of the size orweight of micelles of the sucrose fatty acid esters, treatmentefficiency and other conditions. According to the present invention, bysuitably selecting the fractionation molecular weight of theultrafiltration membrane, it is possible to efficiently removeimpurities including sucrose.

The ultrafiltration membrane should also satisfy the followingconditions.

(1) It has a resistance to a physical external force.

(2) It has a thermal resistance, and is not decomposed bymicroorganisms.

(3) It has an appropriate fractionating molecular weight and has a largetreating ability.

(4) The working life is long.

(5) It is available with an economical cost.

The advance of technique of the preparation of ultrafiltration membranein recent years is marked, and therefore the membranes satisfying theabove conditions are available also from those put on the market.

The washing liquid, namely an aqueous solution containing awater-soluble high HLB-SE (Y), obtained in the prior step, isneutralized with an alkali prior to conducting the ultrafiltration toadjust to a pH of 6.2 to 8.2, preferably a pH in the vicinity of 7.5 inorder to prevent the hydrolysis of SE. When the pH is more than 8.2, thehydrolysis of SE is easy to occur. When the pH is less than 6.2,micelles of the SE are hard to be formed, thus resulting in loss of SEdue to passing through filtration membrane, or resulting in choking ofpores of the membrane.

The thus neutralized aqueous solution is kept at a temperature of nothigher than 80° C. during the ultrafiltration regardless of the kind ofthe fatty acid methyl ester. When the temperature of the aqueoussolution exceeds 80° C., SE may be decomposed. In particular, thehighest filtration velocity is obtained when the temperature of theaqueous solution to be filtered falls within the range of 40° to 60° C.That is to say, sucrose, salt derived from the catalyst such as K.sub.2CO.sub. 3, neutral salt added and volatile material such asdimethylsulfoxide or dimethylformamide pass the most efficiently throughthe filtration membrane with water, when the filtration temperature iskept at 40° to 60° C., especially in the vicinity of 50° C. The reasonis considered that as a result that SE forms huge micelles at atemperature of 40° to 60° C., the total number of micelles decreases andsubstances which do not take part in micelle formation, such as sucrose,become hard to subject to a resistance of SE, thus these substancesbecome easy to move and pass through the membrane.

The SE-containing aqueous solution maintained at a temperature of 40° to60° C. is brought into contact with a ultrafiltration membrane at ahydrogen ion concentration corresponding to a pH of 6.2 to 8.2 under apressure of 1 to 20 kg/cm.sub. 2G applied as actuation source forultrafiltration by a pump.

As stated before, it is important to determine the fractionationmolecular weight of filtration membrane so that the impurities can beseparated efficiently without leakage of SE at a high filtrationvelocity The present inventors have found that membranes having afractionation molecular weight of 1,000 to 100,000 are suitable for thepurposes of the present invention, whereby the purification can becarried out without impairing the separability of sucrose, salts andvolatile material at a high filtration velocity, and particularlymembranes having a fractionation molecular weight of about 5,000 is themost suitable for the treatment on an industrial scale. The filtrationvelocity decreases with decreasing the fractionation molecular weight.On the other hand, when the fractionation molecular weight is high,namely within a range exceeding 5,000 and up to 100,000, the sucroseester may leak. However, such a leakage is slight even if occurs, and isnot economically detrimental.

Cellulose membranes are not much preferred in practical use, since theyare weak against a physical force and are easily attacked withmicroorganisms. Polysulfone and polyvinylidene fluoride membranesreinforced by a support layer are suitable in practical use, and thesefiltration membranes are commercially available. They have excellentthermal, acid and alkali resistances and can withstand a physicalexternal force, and moreover microorganisms do not propagate on themembrane surface. Representative examples of commercially availableultrafiltration membranes suitable for the process of the presentinvention are, for instance, polyvinylidene fluoride membrane (trademark "TERP-E-5") and polysulfone membranes (trade mark "TERP-HF-10 "and"TERP-HF-100") which are sold by Toray Engineering Kabushiki Kaisha.

By the ultrafiltration treatment, impurities such as volatile material,sucrose and salts are removed from the washing liquid obtained in theprior step of washing the precipitate with acidic water, thus a highHLB-SE (Y) having a high purity is recovered usually with theconcentration of a 5 to 15 % by weight aqueous solution. The thuspurified SE contains the monoesters in a high proportion. For example,in case of the crude SE (M.sub. 2) having a monoester content of 73 % byweight shown in FIG. 2, it is divided into a high HLB-SE having amonoester content of 84 % by weight and a low HLB-SE having a monoestercontent of 68 % by weight. Such a high HLB-SE has hitherto not beenprepared easily.

The thus obtained aqueous solution of high HLB-SE can be concentrated upto about 25 % by weight in a usual manner under vacuum, but the solutionform is inconvenient for handling, transportation or the like.Preferably, the high HLB-SE should be in the form of a powder. Spraydrying is optimum for the dehydration of the aqueous SE solution, andhas many advantages as compared with other drying methods. As mentionedbefore, in case of using a usual vacuum dryer as represented by aso-called agitated vacuum dryer, or a flash dryer, deterioration of thequality such as rise in acid value of SE product, coloration or caramelformation is unavoidable due to peculiar viscosity characteristic andlow softening or melting point of SE, and in the latter case, a risk ofdust explosion cannot be disregarded, too. The spray drying according tothe present invention can solve these problems.

In the present invention, the aqueous SE solution is continuously fed toa spray drying tower by a pump, dispersed in the form of mist through anozzle or by a centrifugal force of a rotary disk, and brought intocontact with a dry air stream. Since the surface area of waterevaporation is made extremely large by spray drying, dehydration anddrying can be completed in a very short time, e.g. in several secondsafter spraying. The rotary disc type dryer is preferred from theviewpoint of a high viscosity of the solution.

The SE solution fed to the spray drying tower whose concentration is 4to 40 % by weight is kept at a temperature of 40° to 80° C., preferably40° to 60° C. in consideration of quality. In case of spraying thesolution by means of a rotary disk, the number of rotations thereof isfrom 15,000 to 24,000 r.p.m. when the diameter of the disk is from 5 to10 cm.

The air passed through the tower should have a heat energy sufficient toevaporate water in the solution, and accordingly when the temperature ofthe air is low, a large quantity of air is required as a matter ofcourse. The temperature of the air can be selected from a range of 10°to 100° C., but in consideration of drying efficiency and prevention ofthermal decomposition of the sucrose ester product, the temperature ofthe air is preferably selected from a range of 60° to 80° C.

The humidity of the air passed is also important as well as thetemperature, and it is economical that the absolute humidity of the airis from 0.008 to 0.05 kg water/kg dry air, especially from 0.01 to 0.04kg water/kg dry air.

The parameters such as volume, diameter and height of the spray dryingtower are determined on the basis of the above-mentioned sprayingconditions. Under appropriate conditions, powder of high HLB-SE having awater content of not more than 5 % by weight can be continuously takenout from the lower part of the tower. The obtained dry product is veryexcellent in quality, e.g. color and stench because of short heat time,and also this drying work does not require many hands.

In general, the thus obtained powder contains 0.5 to 5.0 % by weight ofwater, 0.5 to 10.0 % by weight of unreacted fatty acid methyl ester, 0.5to 10.0 % by weight of a soap, 0.5 to 10.0 % by weight of a fatty acidand 98.0 to 80.0 % by weight of high HLB-SE.

Like this, according to the purification technique using water of thepresent invention, the volatile material (reaction solvent) remaining inthe reaction mixture containing SE produced by a reaction of sucrose anda fatty acid alkyl ester can be easily removed without using an organicsolvent as a purification solvent, while sucrose can be easily recoveredin a high purity. Further, from the crude SE from which the volatilematerial and sucrose have been removed, an aqueous solution of purifiedhigh HLB-SE can be easily obtained by washing the crude SE with anacidic water and then subjecting the washing liquid to ultrafiltration.When the concentrate obtained in the ultrafiltration is furtherspray-dried, a dry powder of highly pure high HLB-SE having a goodflowability can be easily, continuously obtained without imparing thequality, e.g. color and stench.

The present invention is more specifically described and explained bymeans of the following Examples in which all % are by weight unlessotherwise noted. It is to be understood that the present invention isnot limited to these Examples.

EXAMPLE 1

A reaction solvent was distilled away in a usual manner from a reactionmixture formed by a reaction of sucrose and methyl stearate according tothe solvent method, and it was neutralized with lactic acid to give aresidue having the composition shown in Table 2.

                  TABLE 2                                                         ______________________________________                                                          Amount                                                      Ingredients         %       kg                                                ______________________________________                                        SE (stearate)*      35.2    35.2                                              Unreacted sucrose   37.5    37.5                                              Unreacted fatty acid methyl                                                                       1.5     1.5                                               ester (methyl stearate)                                                       Potassium lactate   1.2     1.2                                               Soap                2.1     2.1                                               Stearic acid        1.3     1.3                                               Dimethylsulfoxide (DMSO)                                                                          21.2    21.2                                              (reaction solvent)                                                            Total               100.0   100.0                                             ______________________________________                                         (Note):                                                                        *SE had a monoester content of 70% and a di or higherester content of        30%.                                                                     

After drying the residue, to 100 kg of the dried matter was added 1,000kg of water to dissolve it.

To the obtained aqueous solution were added 62.5 kg of sucrose and 97.6kg of a 50 % aqueous solution of potassium lactate and the temperaturewas elevated to 75° C. The resulting precipitate was filtered off togive a cake (water content: 45 %), and it was dried at 80° C. in avacuum. The composition of the dried cake is shown in Table 3.

                  TABLE 3                                                         ______________________________________                                                            Amount                                                    Ingredients           %       kg                                              ______________________________________                                        SE                    79.5    35.0                                            Unreacted fatty acid methyl ester                                                                   3.4     1.5                                             Soap                  4.8     2.1                                             Fatty acid            3.0     1.3                                             Dimethylsulfoxide     2.5     1.1                                             Sucrose               5.8     2.5                                             Others                1.0     0.5                                             Total                 100.0   44.0                                            ______________________________________                                    

Measurement of the filtrate (1,180 kg) according to gel permeationchromatography (GPC) indicated that there was no SE in the filtrate andthat 95 % of dimethylsulfoxide as the reaction solvent was removed.

In 400 kg of an aqueous solution of acetic acid (pH 3.5) having roomtemperature was dispersed 80 kg of the thus obtained cake. Thedispersion was stirred in a homomixer for 10 minutes to wash anduniformly break the precipitate into small particles, and was thenfiltered. This washing operation was repeated four times in total. Thecomposition of the obtained filtrate (pH 3.5) is shown in Table 4.

                  TABLE 4                                                         ______________________________________                                                          Amount                                                      Ingredients         %       kg                                                ______________________________________                                        SE                  66.1    7.8                                               Unreacted fatty acid methyl ester                                                                 1.7     0.2                                               Fatty acid          4.2     0.5                                               DMSO                8.5     1.0                                               Sucrose and others  19.5    2.3                                               Subtotal            100.0   11.8                                              Water               0.0     1,613.2                                           Total               100.0   1,625.0                                           ______________________________________                                    

After adjusting 1,625 kg of the filtrate to pH 7.5 with sodiumhydroxide, it was fed to a spiral type 4 inch cylindrical pressurefiltration unit of membrane area 8 m.sup. 2 equipped with aultrafiltration membrane having a fractionation molecular weight of5,000 (commercially available under the trade mark "TERP-E-5" from TorayEngineering Kabushiki Kaisha) under the following conditions.

Pressure: 7.5 to 9.2 kg/cm.sup. 2G

Temperature: 50.5° to 53.0° C.

Discharge velocity of filtrate: 4.1 to 5.7 kg/8 m.sup. 2.min.

Circulation velocity inside the membrane: 18.9 to 17.3 kg/8 m.sup.2.min.

After about 310 minutes from the start of feeding, to the concentratewhich had not passed through the ultrafiltration membrane was added1,600 kg of water, and it was stirred for dissolution and fed to theabove cylindrical pressure filtration unit again under the sameconditions. This filtration operation was repeated four times in total.The obtained concentrate had a composition (as the dried matter) shownin Table 5 and a solid content of 11.5 %.

                  TABLE 5                                                         ______________________________________                                                        Amount                                                        Ingredients       %         kg                                                ______________________________________                                        SE                91.6          7.7                                           Unreacted fatty acid                                                                            2.4           0.2                                           methyl ester                                                                  Fatty acid        6.0           0.5                                           DMSO              (1.1   ppm)   --                                            Sucrose and others                                                                              0.0           0.0                                           Total             100.0         8.4                                           ______________________________________                                    

The concentrate having the composition shown in Table 5 was adjusted topH 7.5. The thus pH-adjusted concentrate (aqueous solution) had acomposition (as the solid matter) shown in Table 6.

                  TABLE 6                                                         ______________________________________                                                        Amount                                                        Ingredients       %         kg                                                ______________________________________                                        SE*               91.6          7.7                                           Unreacted fatty acid                                                                            2.4           0.2                                           methyl ester                                                                  Soap              4.1           0.34                                          Fatty acid        1.9           0.16                                          DMSO              (1.1   ppm)   --                                            Total             100.0         8.4                                           ______________________________________                                         (Note)                                                                        *SE had a monoester content of 83.2% and a di or higherester content of       16.8%.                                                                   

The pH-adjusted solution was heated in a vaccum to concentrate. Theobtained concentrate contained 24 % of a high HLB-SE having a monoestercontent of about 83 %.

On the other hand, the solid residue obtained by washing the cake havingthe composition shown in Table 3 with acetic acid contained SE having amonoester content of 67 %, namely was a low HLB-SE.

EXAMPLE 2

The procedure of Example 1 was repeated to give the concentrate havingthe composition (as solid matter) shown in Table 6 and containing 24 %of a high HLB-SE having a monoester content of about 83 %.

The obtained concentrate was fed into a spray drying tower under thefollowing conditions.

Diameter of spray drying tower: 2.0 m

Length of true cylindrical portion: 1.5 m

Fed air: 350 N m.sub. 3/hour

Diameter of rotary disk: 10 cm

Number of rotations of the disk: 24,000 r.p.m.

Temperature of air at inlet: 51° C.

Absolute humidity of air at inlet: 0.020 kg water/kg dry air

Feeding velocity of concentrate: 1.1 kg/hour

The powdery high HLB-SE obtained from the lower part of the spray dryingtower had a water content of 2.10 %, a bulk specific gravity of 0.41, nocoloration due to heating and a good flowability. The composition of theobtained powder is shown in Table 7. The drying was continued stably for1.5 hours.

                  TABLE 7                                                         ______________________________________                                        Ingredients         Amount (%)                                                ______________________________________                                        SE*                 91.6                                                      Unreacted fatty acid methyl ester                                                                 2.4                                                       Soap                4.2                                                       Fatty acid          1.8                                                       DMSO                (1.0      ppm)                                            Sucrose             0.0                                                       Total               100.0                                                     ______________________________________                                         (Note)                                                                        *SE had a monoester content of 83.1% and a di or higherester content of       16.9%.                                                                   

As shown above, from the SE having the monoester content of 70 % (di- orhigher-ester content: 30 %) in the original reaction mixture, the highHLB-SE having the monoester content of 83.1 % could be obtained. The SEleft as the solid in washing of the cake shown in Table 3 with aceticacid was low HLB-SE having a monoester content of 67 % as mentionedabove. This fact indicates that the SE in the original reaction mixturecould be fractionated into the high HLB-SE and the low HLB-SE.

EXAMPLE 3

An aqueous solution of the dried residue having the composition shown inTable 2 was prepared, salted out and filtered in the same manner as inExample 1 to give the cake of precipitate and the filtrate.

While the obtained cake having the composition shown in Table 3 wastreated by washing with acidic water, ultrafiltration and spray-dryingin the same manner as in Examples 1 and 2 to give a powder of highHLB-SE having the composition shown in Table 7, the above filtrate wastreated in order to recover sucrose as follows:

Water was added to 1,180 kg of the filtrate (aqueous solution containingsucrose, salts and volatile material, obtained by removing SEprecipitated by the above-mentioned salting-out) to give an aqueoussolution having the composition shown in Table 8.

                  TABLE 8                                                         ______________________________________                                                        Amount                                                        Ingredients       %       kg                                                  ______________________________________                                        SE                0.01    0.2                                                 Sucrose           5.83    97.5                                                Potassium lactate 2.96    49.5                                                DMSO              1.20    20.1                                                Subtotal          10.00   167.3                                               Water             90.00   1505.7                                              Total             100.00  1673.0                                              ______________________________________                                    

The aqueous solution (pH 7.4) was heated at a temperature of 50° to52.5° C., and was fed to a reverse osmosis membrane "SU-200"(Toray'strade mark) having a diameter of 4 inches, a length of 1 m and afiltrating area of 8 m.sup. 2 at a pump pressure of 58.2 kg/cm.sup. 2Gunder the following operation conditions.

Discharge velocity of the aqueous solution passed through the reverseosmosis membrane: 3.9 to 2.2 l/minute

Circulation velocity inside the membrane: 19.2 to 20.9 l/minute

Feeding time: about 550 minutes

The concentrate which had not passed through the membrane containedsucrose, the salt derived from the catalyst and the volatile material inamounts of almost whole, 46.0 % and 52.0 % of those included in theoriginal reaction mixture, respectively. The composition of theconcentrate is shown in Table 9.

On the other hand, the aqueous solution which had passed through themembrane, namely the filtrate, as shown in Table 9, scarcely containedsucrose, and contained the salt derived from the catalyst, the neutralsalt and the volatile material in amounts of 54.0 % and 48.0 % of thoseincluded in the original solution to be treated, respectively.

                  TABLE 9                                                         ______________________________________                                                  Filtrate        Concentrate                                         Ingredients kg      %         kg     %                                        ______________________________________                                        SE          0.0     0.0       0.2    0.2                                      Sucrose     0.0     0.0       97.5   74.4                                     Potassium lactate                                                                         26.7    73.5      22.8   17.4                                     DMSO        9.6     26.5      10.5   8.0                                      Subtotal    36.3    100.0     131.0  100.0                                    Water       588.7             917.0                                           Total       625.0             1048.0                                          ______________________________________                                    

EXAMPLE 4

To 1,048 kg of the concentrate (solute concentration: 12.5 %) having thecomposition shown in Table 9, obtained in Example 3 was added 1,900 kgof water, and the thus obtained aqueous solution was fed to the reverseosmosis membrane under the same conditions as in Example 3 to isolatesucrose.

The results are shown in Table 10.

                  TABLE 10                                                        ______________________________________                                                 Filtrate         Concentrate                                         Ingredients                                                                              kg       %         kg     %                                        ______________________________________                                        SE         0.0      0.0       0.2    0.02                                     Sucrose    0.0      0.0       97.5   96.13                                    Potassium lactate                                                                        15.7     68.2      7.1    0.70                                     DMSO       7.3      31.8      3.2    3.15                                     Subtotal   23.0     100.0     108.0  100.00                                   Water      1909.8             907.2                                           Total      1932.8             1015.2                                          ______________________________________                                    

EXAMPLE 5

To 1,015.2 kg of the concentrate (solute concentrate: 10.6 %) shown inTable 10, obtained in Example 4 was added 2,000 kg of water and the thusobtained aqueous solution was fed into the reverse osmosis membraneunder the same conditions as in Example 3 to isolate sucrose. Theresults are shown in Table 11.

                  TABLE 11                                                        ______________________________________                                                 Filtrate         Concentrate                                         Ingredients                                                                              kg       %         kg     %                                        ______________________________________                                        SE         0.0      0.0       0.1    0.1                                      Sucrose    0.0      0.0       97.4   97.0                                     Potassium lactate                                                                        5.3      71.6      1.8    1.8                                      DMSO       2.1      28.4      1.1    1.1                                      Subtotal   7.4      100.0     100.4  100.0                                    Water      2197.4             910.0                                           Total      2204.8             1010.4                                          ______________________________________                                    

In addition to the ingredients used in the Examples, other ingredientscan be used in the Examples as set forth in the specification to obtainsubstantially the same results.

What we claim is:
 1. A process for purifying sucrose fatty acid estershaving a high hydrophilic-lipophilic balance included in a reactionmixture formed by a reaction of sucrose and a fatty acid alkyl ester inan organic solvent, said reaction mixture containing sucrose fatty acidesters, unreacted sucrose, unreacted fatty acid alkyl ester, a reactioncatalyst, a soap, a fatty acid and the organic solvent as a volatilematerial, which comprises adjusting the reaction mixture to a neutral pHregion, adding water, a neutral salt and sucrose to the reaction mixtureto precipitate the sucrose fatty acid esters, separating the resultingprecipitate from the liquid phase, washing it with an acidic water,neutralizing the washing liquid and subjecting it to ultrafiltration. 2.The process of claim 1, wherein said reaction mixture to be treatedconsists essentially of 15.0 to 95.0 % by weight of sucrose fatty acidesters, 1.0 to 80.0 % by weight of unreacted sucrose, 0.5 to 10.0 % byweight of unreacted fatty acid methyl ester, 0.05 to 7.0 % by weight ofa catalyst, 1.0 to 10.0 % by weight of a soap, 0.5 to 10.0 % by weightof a fatty acid and 3.0 to 30.0 % by weight of a remaining reactionsolvent.
 3. The process of claim 1, wherein said neutral pH region isfrom pH 6.2 to pH 8.2.
 4. The process of claim 1, wherein after theaddition of water, neutral salt and sucrose, the reaction mixture isheated to a temperature of 50° to 80° C.
 5. The process of claim 1,wherein said water is added to the reaction mixture in a water/reactionmixture ratio of 5 : 1 to 40 : 1 by weight.
 6. The process of claim 1,wherein said neutral salt and sucrose are added to the reaction mixturein amounts satisfying the following questions: ##EQU6## wherein TSameans the sum of the neutral salt to be added and a salt formed byneutralization of catalyst used in the synthesis of sucrose fatty acidester, TSu means the sum of sucrose to be added and the unreactedsucrose present from the beginnng, and TH.sub. 2O means the amount ofwater present.
 7. The process of claim 1, wherein the pH adjustment ofthe reaction mixture is made with an acid selected from the groupconsisting of lactic acid, acetic acid, hydrochloric acid and sulfuricacid.
 8. The process of claim 1, wherein the fatty acid radical includedin each of the fatty acid alkyl ester, soap and fatty acid is asaturated fatty acid radical having 16 to 22 carbon atoms.
 9. Theprocess of claim 1, wherein said volatile material is dimethylsulfoxideor dimethylformamide.
 10. The process of claim 1, wherein said neutralsalt to be added to the reaction mixture is a salt selected from thegroup consisting of sodium chloride, Glauber's salt, potassium lactateand potassium acetate.
 11. The process of claim 1, wherein said sucrosefatty acid ester is composed of 10 to 75 % by weight of the monoesterand 90 to 25 % by weight of the di- and higher esters.
 12. The processof claim 1, wherein said acidic water has a pH of 3.0 to 5.5.
 13. Theprocess of claim 1, wherein the water is maintained at a temperature of10° to 40° C.
 14. The process of claim 1, wherein the membrane used forthe ultrafiltration is made of a polysulfone or a polyvinylidenefluoride.
 15. The process of claim 1, wherein the membrane for theultrafiltration has a fractionation molecular weight of 1,000 to100,000.
 16. The process of claim 1, wherein said ultrafiltration iscarried out under a pressure of 1.0 tp 20.0 kg/cm.sup. 2G.
 17. Theprocess of claim 1, wherein the washing liquid is neutralized to pH 6.2to 8.2.
 18. The process of claim 1, wherein the washing liquid subjectedto ultrafiltration is maintained at a temperature of 40° to 60° C. 19.The process of claim 1, wherein the liquid phase obtained by separatingthe precipitate is subjected to reverse osmosis, thereby recoveringsucrose.
 20. The process of claim 19, wherein a membrane used for thereverse osmosis has a fractionation molecular weight of 150 to
 200. 21.The process of claim 19, wherein said liquid phase is maintained at atemperature of 40° to 60° C. and at a pH of 6.2 8.2.
 22. The process ofclaim 19, wherein said liquid phase has a sucrose content of 10 to 20 %by weight.
 23. The process of claim 1, wherein the concentrate obtainedby the ultrafiltration is spray-dried.
 24. The process of claim 23,wherein said concentrate has a solid content of 4 to 40 % by weight. 25.The process of claim 24, wherein said spray-drying is conducted in anair stream having an absolute humidity of 0.008 to 0.05 kg water/kg dryair and a temperature of 10° to 100° C.